"The 38-year-old Perth man, who has a weakened immune system, initially responded to the drug but developed a resistant strain of the virus when his illness relapsed," the state's Department of Health said in a statement.

There have been 13 cases of Tamiflu-resistant infections around the world, the statement said.

A Roche executive said on Monday that isolated cases of Tamiflu-resistant H1N1 pandemic flu were to be expected, in line with what has been seen in clinical studies.

"There is no evidence that the virus has spread to other people. None of the patient's family or hospital staff caring for him have contracted the virus, and he has not been in contact with the wider community," said the state's Chief Health Officer Tarun Weeramanthri, adding that the case was a rare and isolated one.